The present invention relates to a connection structure in which an electronic component is connected to a board by flow soldering, and an electronic circuit board that includes the connection structure.
As a conventional process for manufacturing an electronic circuit board by connecting an electronic component(s) onto a board (or a substrate) such as a printed circuit board, there is a process that employs the so-called flow soldering. FIG. 13 shows a schematic partial sectional view of an electronic circuit board manufactured by the conventional flow soldering process. FIG. 14(b) shows an enlarged view of a part shown in FIG. 13, and FIG. 14(a) shows a schematic plan view from top of the part shown in FIG. 14(b) while excluding a fillet.
According to the conventional flow soldering process in a general way, as shown in FIG. 13 and FIG. 14, first a lead 65 (for example, an electrode) extending from an electronic component 67 is inserted to a through hole 62 formed in a board 61 to penetrate therethrough from the front surface (top in the drawing) to the back surface (bottom in the drawing) of the board 61. The board 61 has a land 63 (FIG. 13) made of, for example, a copper foil. The land 63 is formed along a wall surface of the through hole 62 and on front and back surfaces of the board 61 around the through hole 62, and portions of the land 63 on these three surfaces are called a wall surface land portion 63c, a front surface land portion 63a and a back surface land portion 63b, respectively (FIG. 14(b)). The land 63 is connected to a wiring pattern (not shown) formed on the front surface or the back surface of the board 61. The board 61 is covered by a solder resist 64 (indicated with hatching in FIG. 13) on the front surface and back surface thereof except for the land 63.
Then a solder material that has been molten by heating is applied in the form of a wave(s) to the board 61 from the back surface of the board 61. The molten solder material rises up an annular space between the through hole 62 and the lead 65 inserted to the through hole 62 (see, FIG. 14(a)), and spreads over the front surface land portion 63a and the back surface land portion 63b while wetting the surface thereof. The solder material solidifies as its temperature lowers, thereby forming a connection portion 66 as shown in FIG. 13 and FIG. 14(b) (not shown in FIG. 14(a)). The solder material does not deposit on the surface region that is covered by the solder resist 64.
The connection portion 66 made of the solder material, the so-called fillet, is formed as described above (hereinafter, it is simply referred to as the fillet).
Thus the lead 65 of the electronic component 67 and the land 63 formed on the board 61 are electrically and mechanically connected thereby making an electronic circuit board 70.
In the past, the electronic circuit board manufactured as described above generally employs an Snxe2x80x94Pb based solder material that contains Sn and Pb as main components, particularly an Snxe2x80x94Pb eutectic solder material. However, lead included in the Snxe2x80x94Pb based solder material has a possibility of causing environmental pollution if it is wasted improperly. Accordingly, as an alternative of such lead-containing solder material, a solder material containing no lead, the so-called lead-free solder material has begun to be used on an industrial scale. However, use of the lead-free solder material in the flow soldering process for manufacturing the electronic circuit board leads to such a problem that an upper fillet portion 66a and/or a lower fillet portion 66b peels off (or is lifted off) the front surface land portion 63a and/or the back surface land portion 63b, respectively, as shown in FIG. 14(b), resulting in insecure connection between the land and the fillet. This phenomenon, which is generally called xe2x80x9clift-offxe2x80x9d, is undesirable for an electronic circuit board that is required to have high bonding strength between the lead of the electronic component and the land in order to provide high reliability of the electronic circuit board. The lift-off hardly occurs in a case of using the Snxe2x80x94Pb eutectic solder material and there has been no concern over the problem, but recent finding shows that the lift-off frequently occurs in a case of using the lead-free solder material thus raising a concern about it. As the transition from the conventional Snxe2x80x94Pb based solder material to the lead-free solder material is promoted, it is very important in the manufacture of electronic circuit boards to prevent the lift-off that is a characteristic problem accompanying the use of the lead-free solder material.
The lift-off that occurs when using the lead-free solder material is generally supposedly to be arisen by following two types of cause.
One type of cause relates to the composition of the lead-free soldering material itself used in flow soldering. When using an Snxe2x80x94Agxe2x80x94Bi based alloy, for example, as the solder material, an Snxe2x80x94Bi eutectic solder material among all of metal elements (Sn, Ag, Bi) that constitute the Snxe2x80x94Agxe2x80x94Bi based alloy and all of alloys (an Snxe2x80x94Ag based alloy, an Snxe2x80x94Bi based alloy and an Agxe2x80x94Bi based alloy) that can be formed from any combination of these metal elements has a melting point (about 138xc2x0 C.) that is lower than a melting point (about 200xc2x0 C.) of the original Snxe2x80x94Agxe2x80x94Bi based alloy. A metal and/or an alloy that has a melting point lower than the melting point of the original alloy will be hereafter referred to simply as a low-melting point metal or a low-melting point alloy. In the case of the Snxe2x80x94Agxe2x80x94Bi based alloy, the Snxe2x80x94Bi eutectic alloy is the low-melting point metal.
In this case, as the Snxe2x80x94Agxe2x80x94Bi based alloy supplied in a molten state to the board gradually solidifies, the Snxe2x80x94Bi eutectic alloy that has a melting point lower than that of the original Snxe2x80x94Agxe2x80x94Bi based alloy migrates to a region not yet solidified due to the temperature gradient in the fillet, and is concentrated at the region. As a result, the Snxe2x80x94Bi eutectic alloy is segregated by concentration toward the hottest region of the fillet, namely a portion that solidifies last. The hottest region in an upper portion of the fillet (i.e. an upper fillet portion) is a region near an interface between the land made of copper foil that is a good heat conductor and the fillet (hereafter also referred to simply as a land/fillet interface), and the low-melting point alloy 75 such as the Snxe2x80x94Bi eutectic alloy is concentrated in the vicinity of the land/fillet interface as shown in FIG. 14(b) to segregate. In the process of solidification of the fillet, in case a tension (indicated schematically by arrow of dashed line for only the upper fillet portion 66a in FIG. 14(b)) due to shrinkage by solidification is exerted in the low-melting point alloy near the land/fillet interface on the board surface where the solder material is still molten and has an insufficient strength, cracks are generated at a periphery of the land/fillet interface, and proceed toward an inside (or a central portion) of the fillet 66a and 66b as the solidification proceeds from the periphery to the inside. Since the cracks are generated in the fillet as described above, a peripheral edge 71 of the fillet is supposed to peel off a peripheral edge of the land 72, as shown in FIG. 14(b), thus causing the lift-off.
Another type of cause relates to combination of a material of the lead 65 and the solder material of the fillet 66. In case the molten solder material that makes contact with the lead 65 is made of an Sn-0.7Cu eutectic alloy (an alloy formed from 0.7% by weight of Cu and the balance of Sn), for example, there is no low-melting point metal or low-melting point alloy having a melting point that is lower than a melting point (about 227xc2x0 C.) of the original Snxe2x80x94Cu eutectic alloy among all of the metal elements constituting the solder material and all of the alloys that can be formed from any combination of these metal elements, and therefore the cause described above does not work in this case. However, such cracks as described above are generated also in this case and the lift-off can occur. This may occur depending on combination of the material of the lead, for example the material of a plating member of the lead, and the solder material used in the flow soldering.
The lead 65 generally consists of a base member and a plating member covering the base member (hereafter referred to simply as a plating member). The lead is typically plated with an Snxe2x80x94Pb based material. In this case, the solder material makes contact with the plating member when the molten solder material of high temperature rises up the through hole 62, so that a component of the plating member can melt into the solder material. Since the Snxe2x80x94Pb eutectic alloy has a melting point (about 183xc2x0 C.) that is lower than the melting point of the Snxe2x80x94Cu eutectic alloy (about 227xc2x0 C.) used as the solder material, the Snxe2x80x94Pb eutectic alloy having the lower melting point can segregate in the vicinity of the land/fillet interface when the molten Snxe2x80x94Cu eutectic alloy solidifies through a process similar to that described above, thus resulting in lift-off.
In addition to the combination of the Snxe2x80x94Cu based solder material and the Snxe2x80x94Pb based plating material, A low-melting point alloy such as Snxe2x80x94Bi (melting point: about 138xc2x0 C.), Snxe2x80x94Zn (melting point: about 199xc2x0 C.) or Snxe2x80x94In (melting point: about 118xc2x0 C.) can also be formed, for example in a case a lead-free solder material of Snxe2x80x94Cu (melting point: about 227xc2x0 C.), Snxe2x80x94Agxe2x80x94Cu (melting point: about 220xc2x0 C.) or the like that is commonly used in flow soldering is combined with a plating material that includes a metal element of Bi, Zn or In. Lift-off occurs in such a case as described above where there is a metal element, which has a melting point lower than that of the original soldering material, among all of the metal elements that constitute the lead (for example the plating member) as well as the solder material and all of the alloys that can be formed from all possible combinations of two or more of these metal elements.
Both of the two causes arise from the fact that a low-melting point alloy (or low-melting point metal) having a composition different from the composition of the alloy constituting a bulk of the fillet (the latter composition is substantially same as the composition of the original solder material) precipitates in the vicinity of the land/fillet interface. Furthermore, the two causes may act in combination in actual fact.
It is supposed that any of the two causes described above does not exist in a combination of the conventional Snxe2x80x94Pb eutectic solder material and the Snxe2x80x94Pb plating material, thus the problem of lift-off does not arise. Lift-off occurs conspicuously when the lead-free solder material is used, and therefore may be regarded as a distinctive problem of the lead-free solder material.
The present invention has been made in order to solve the problem of the conventional constitution described above. An object of the present invention is to provide a connection structure comprising a board with a electronic component connected thereon by flow soldering wherein the occurrence of lift-off is effectively reduced, and an electronic circuit board that includes the connection structure.
The present inventors have studied for effectively reducing the occurrence of lift-off in the flow soldering process while approaching in view of structure and/or material of the connection structure, and thus the present invention has been completed.
In one aspect of the present invention, there is provided a connection structure comprising a board having a front surface and a back surface with a through hole formed therein; a land including a wall surface land portion formed on a wall surface of the through hole and front and back surface land portions respectively formed on the front and back surfaces of the board around the through hole; a lead extending from an electronic component and disposed so as to penetrate the through hole from the front surface to the back surface of the board; and a fillet made of a solder material by flow soldering so as to connect the land and the lead, the fillet including upper and lower fillet portions respectively located on the front and back surfaces of the board, wherein a profile of the upper fillet portion contacting with the front surface land portion is smaller than a profile of the lower fillet portion contacting with the back surface land portion, and is not smaller than a profile of the through hole.
As used herein, the profile of the upper fillet portion or the lower fillet portion refers to an outline (or a contour) or a sectional area of the fillet in a section that is substantially parallel to the principal plane (or the front surface or the back surface) of the board where the sectional area is maximum, and is generally equal to an outline of an exposed portion of the front surface land portion or the back surface land portion whereon the molten solder material has been supplied by flow soldering and spreads, or to an area wetted by the solder material. The profile of the upper fillet portion or the lower fillet portion is typically represented by an outer diameter thereof or an outer diameter of the exposed portion in case the exposed portion of the front surface land portion or the back surface land portion has an annular shape (and therefore has a circular outline). As used herein, the exposed portion of the front surface land portion or the back surface land portion refers to a portion that is exposed when the solder material is supplied in the flow soldering process and is to make contact with the solder material.
In the connection structure of the present invention as described above, since an outer diameter of the upper fillet portion is smaller than that of the lower fillet portion, an exposed sloping area of the upper fillet portion can be made smaller than in the case where the outer diameter of the upper fillet portion is larger than that of the lower fillet portion, thus making it possible to decrease the tension generated by the solidification and shrinkage of the solder material. This makes it possible to make the possibility of lift-off, that conspicuously occurs when the lead-free solder material is used, effectively lower than in the conventional constitution.
In one embodiment of the present invention, a profile of the front surface land portion is smaller than a profile of the back surface land portion and/or a peripheral edge of the front surface land portion is covered by a solder resist. By forming the land itself so that the outer diameter of the front surface land portion is smaller than the outer diameter of the back surface land portion, the outer diameter of the upper fillet portion can be made smaller than the outer diameter of the lower fillet portion. The outer diameter of the upper fillet portion can also be made smaller than the outer diameter of the lower fillet portion by covering the peripheral edge of the front surface land portion with the solder resist, so that the outer diameter of the exposed portion of the front surface land portion is smaller than the outer diameter of the front surface land portion itself and making the outer diameter of the exposed portion of the front surface land portion smaller than the outer diameter of the back surface land portion. It is noted that the profile of the front surface land portion or the back surface land portion refers to an outline of the land as formed on the front surface or the back surface of the board or an area of the land on a plane which is parallel to the front surface or the back surface.
In a preferred embodiment, the wall surface land portion has a tapered portion at an end thereof connecting to the front surface of the board. By providing such tapered portion, a region of the highest temperature of the solder material is formed in a vicinity of the tapered portion in the process of solidification, so that the low-melting point metal can be concentrated in such region while preventing from precipitating near the peripheral edge of the filet. Furthermore, such a tapered portion makes it possible to improve the bonding strength between the fillet and the land so that the connection between the fillet and the land can withstand the tension caused by the solidification and shrinkage of the solder material. This further reduces the probability of the occurrence of lift-off.
In a preferred embodiment, another through hole is provided to penetrate the board at a position near the through hole described above, while the land is formed so as to cover a wall surface of the another through hole and areas intervening between these two through holes on the front surface and the back surface of the board respectively. The areas intervening between the through holes on the front surface of the board is covered with the solder resist. In such a constitution, a temperature of the solder material becomes highest on the intervening land portions between the through holes during the solidification process, so that the low-melting point metal has a positive tendency to segregate in the vicinity of the intervening land portions between the two through holes, thus an amount of low-melting point metal precipitating in the vicinity of the other land/fillet interface can be made relatively smaller. This makes it possible to localize the occurrence of cracks upon the intervening land portions between through holes so that, even when lift-off occurs locally, it is confined in a very limited region, namely in the vicinity of the intervening land portions between the through holes and is made less likely to occur in other region, thereby preventing lift-off from occurring completely.
In another aspect of the present invention, there is provided a connection structure wherein temperature rise in the front surface land portion is suppressed instead of making the profile of the upper fillet portion that contacts the front surface land portion smaller than the profile of the lower fillet portion that contacts the back surface land portion and not smaller than the profile of the through hole, or in addition thereto. Specifically, there is such an aspect as described below.
In one embodiment of the present invention, the profile of the upper fillet portion that makes contact with the front surface land portion is smaller than outer diameter of the front surface land portion. This makes it possible to dissipate an amount of heat of the solder material through a region of the front surface land portion that does not make contact with the upper fillet portion, so that the temperature of the front surface land portion is decreased thereby suppressing the temperature rise and the amount of low-melting point metal precipitating in this region can be decreased. As a result, it is made possible to make the probability of the occurrence of lift-off, that conspicuously occurs when the lead-free solder material is used, effectively lower than in the conventional constitution. For example, the profile of the upper fillet portion can be made smaller than the profile of the front surface land portion by applying flow soldering to the board whereon the peripheral edge of the front surface land portion has been covered with the solder resist in advance.
In another embodiment of the present invention, the board includes a heat sink formed in contact with the front surface land portion. In this case, since an amount of heat transmitted from the solder material to the front surface land portion can be dissipated to the heat sink, the temperature rise in the front surface land portion can be suppressed similarly to the embodiment described above, thus makes it possible to make the probability of the occurrence of lift-off, that conspicuously occurs when the lead-free solder material is used, effectively lower than in the conventional structure. The heat sink may be formed by embedding in the board followed by forming the land so as to make contact the front surface land portion with the heat sink.
In further another embodiment of the present invention, a notch is formed in the wall surface land portion. When the solder material cools down through heat dissipation and solidification proceeds from an exposed portion of the solder material, the filet portion between the upper fillet portion and the lower fillet portion is at the highest temperature as to the entire fillet. In the conventional structure, an amount of heat of this fillet portion made of the solder material is transmitted through the wall surface land portion to the front surface land portion resulting in high temperature in the vicinity of a front surface land portion/fillet interface. When the notch is provided in the wall surface land portion as in this embodiment, in contrast, supply of heat from the fillet portion between the upper fillet portion and the lower fillet portion through the wall surface land portion is shut off, thus making it possible to cool down the front surface land portion faster than in the case of the conventional structure and suppress the temperature rise in the front surface land portion. Thus it is made possible to make the amount of the low-melting point metal precipitating in this region smaller and makes it possible to make the probability of the occurrence of lift-off, that conspicuously occurs when the lead-free solder material is used, effectively lower than in the conventional structure.
In another aspect of the invention, there is provided a connection structure wherein at least one projection is provided on at least one of the front surface land portion and the back surface land portion instead of instead of making the profile of the upper fillet portion that makes contact with the front surface land portion smaller than the profile of the lower fillet portion that makes contact with the back surface land portion and not smaller than the profile of the through hole or in addition thereto. According to this embodiment, since the projection is provided in the front surface land portion and/or the back surface land portion, propagation of a crack from the peripheral edge of a land/fillet interface towards the lead can be stopped by the projection, thereby improving the connection strength between the land and the filet. As a result, the connection between the fillet and the land can withstand the tension caused by the solidification and shrinkage of the solder material, thus making it possible to effectively reduce the occurrence of lift-off, that conspicuously occurs when the lead-free solder material is used.
In one embodiment of the invention, the projection is located on a peripheral edge of at least one of the front surface land portion and the back surface land portion. According to this embodiment, since the projection is provided on the peripheral edge of the front surface land portion and/or the back surface land portion, crack can be caused to propagate in downward direction along a projection/fillet interface instead of the direction from the peripheral edge of the land toward the lead in the land/fillet interface, thus making it possible to improve the connection strength between the land and the fillet. As a result, the connection between the fillet and the land can withstand the tension caused by the solidification and shrinkage of the solder material, thus making it possible to effectively reduce the possibility of the occurrence of lift-off, that conspicuously occurs when the lead-free solder material is used.
In another aspect of the invention, there is provided a connection structure wherein a portion of the lead that is not inserted into the through hole and is located above the front surface of the board is covered by the solder resist so as to restrict a height of the fillet from the front surface of the board by the solder resist, instead of making the profile of the upper fillet portion that makes contact with the front surface land portion smaller than the profile of the lower fillet portion that makes contact with the back surface land portion and not smaller than the profile of the through hole or in addition thereto. According to this embodiment, since the height of the fillet is restricted by the solder resist, a direction of tension generated by the solidification and shrinkage of the solder material at the peripheral edge of an interface of the upper fillet portion/the front surface land portion comes to cross the surface of the board with a more acute angle, thereby decreasing a vertical component of the tension so that peeling of the fillet off the land/fillet interface is less likely to occur. This makes it possible to effectively reduce the possibility of the occurrence of lift-off, that conspicuously occurs when the lead-free solder material is used.
In another aspect of the invention, there is provided a connection structure wherein the land is covered by a metal film, instead of making the profile of the upper fillet portion that makes contact with the front surface land portion smaller than the profile of the lower fillet portion that makes contact with the back surface land portion and not smaller than the profile of the through hole, or in addition thereto. According to this aspect, since the land, preferably the land as a whole, is covered by the metal film so as not to make contact with the solder material, a diffusion layer is formed between the metal film and the land wherein the diffusion layer does not dissolve into the molten solder material during flow soldering, thereby preventing the low-melting point metal from segregating in the land/fillet interface. Thus makes it possible to make the possibility of the occurrence of lift-off, that conspicuously occurs when the lead-free solder material is used, effectively lower than in the conventional constitution. The metal film is preferably made of a metal selected from the group consisting of Sn, Snxe2x80x94Cu and Snxe2x80x94Ag, for example a leveler made of such a material.
The connection structures having the various structures according to the present invention as described above may be used individually or in combination thereof.
According to further another aspect of the present invention, there is provided a connection structure comprising a board having a front surface and a back surface with a through hole formed to penetrate through the board; a land formed on a wall surface of the through hole and on the front and back surfaces of the board around the through hole; a lead extending from an electronic component and disposed in the through hole; and a fillet formed from a solder material by flow soldering so as to connect the land and the fillet, wherein all of metal elements that constitute the solder material and all of alloys that can be formed from two or more metal elements selected from the group consisting of these metal elements have melting points not lower than that of the solder material.
In the connection structure of the present invention described above, since there is no low-melting point metal or low-melting point alloy which has a melting point lower than that of the original solder material, among all of metal elements that constitute the solder material and the all of alloys that can be formed from two or more of these metal elements in all possible combinations. This removes one of the causes of lift-off occurring due to the composition of the solder material itself used in the flow soldering. Thus, it is made possible to make the possibility of the occurrence of lift-off, that conspicuously occurs when the lead-free solder material is used, effectively lower than in the conventional structure.
In a preferable embodiment, the lead includes a base member and a plating member that covers the base member, while all of metal elements that constitute the solder material and a material of the plating member, and all of alloys that can be formed from two or more metal elements selected from the group consisting of these metal elements have melting points not lower than that of the solder material. In this case, the other cause of lift-off occurring due to the combination of the composition of the material of the plating member of the lead and the composition of the solder material used in the flow soldering can be eliminated, thus making it possible to reduce the occurrence of lift-off further.
In a further preferable embodiment, all of metal elements that constitute the solder material, the material of the base member and a material of the plating member, and all of alloys that can be formed from two or more of the metal elements selected from the group consisting of these metal elements have melting points not lower than that of the solder material. In this case, occurrence of lift-off can be reduced further by taking the composition of the material of the base member of the lead as well as the solder material and the material of the plating member into consideration.
The solder material is preferably a lead-free solder material selected from the group consisting of Snxe2x80x94Cu, Snxe2x80x94Agxe2x80x94Cu, Snxe2x80x94Ag and Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu. The material of the plating member is preferably a metal selected from the group consisting of Sn, Snxe2x80x94Cu and Snxe2x80x94Ag. The material of the base member is preferably a metal selected from the group consisting of Cu, Fe and Fexe2x80x94Cu. Preferable combinations of the solder material/the material of the base member/the material of the plating member include, for example, Snxe2x80x94Cu/Snxe2x80x94Cu/Cu, Snxe2x80x94Cu/Sn/Cu, Snxe2x80x94Cu/Snxe2x80x94Cu/Fe, Snxe2x80x94Cu/Sn/Fe, Snxe2x80x94Cu/Snxe2x80x94Ag/Cu and Snxe2x80x94Cu/Snxe2x80x94Ag/Fe.
In another preferable embodiment, the lead includes the base member and the plating member that covers the base member, while at least one of the metal elements that constitute the base member and/or at least one of alloys formed from: a metal element(s) selected from the metal elements that constitute the solder material and the plating member; and the metal element(s) that constitute the base member has melting point lower than that of the solder material, and the lead is further includes means or a structure for preventing the metal elements that constitute the base member from melting into the fillet. In case the base member is made of a Zn based alloy, for example, the means described above may be an underlaying plating member made of Ni disposed between the base member and the plating member. In this case, Zn that constitutes the base member does not melt into the solder material, so that the formation of a low-melting point metal can be avoided. This makes it possible to make the possibility of the occurrence of lift-off, that conspicuously occurs when the lead-free solder material is used, effectively lower than in the conventional constitution.
The feature related to the material of the connection structure of the present invention described above are preferably used in combination with the various connection structures of the present invention that have the structural features described above, thereby making it possible to make the possibility of lift-off, that conspicuously occurs when the lead-free solder material is used, effectively lower than in the conventional constitution.
All connection structures of the present invention described above can be preferably used for an electronic circuit board that includes a connection structure wherein electronic components are connected onto a board.
Also all connection structures of the present invention are preferably used for a flow soldering process of electronic components onto a board using a lead-free solder material. In this specification, the lead-free solder material refer to a solder material that does not substantially include lead, namely a solder material having lead content that is typically 0.1% by weight or less. Lead-free solder materials include solder materials such as Snxe2x80x94Cu, Snxe2x80x94Agxe2x80x94Cu, Snxe2x80x94Ag, Snxe2x80x94Agxe2x80x94Bi, Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu and Snxe2x80x94Agxe2x80x94Bixe2x80x94In.
The present invention includes the following embodiments 1 to 33.
(Embodiment 1) A connection structure comprising a board having a front surface and a back surface with a through hole formed to penetrate through the board; a land including a wall surface land portion formed on a wall surface of the through hole, a front surface land portion formed on the front surface of the board around the through hole and a back surface land portion formed on the back surface of the board around the through hole; a lead extending from an electronic component and disposed so as to penetrate the through hole from the front surface to the back surface of the board; and a fillet made of a solder material by flow soldering so as to connect the land and the lead, the fillet including an upper fillet portion located on the front surface of the board and a lower fillet portion located on the back surface of the board, wherein a profile of the upper fillet portion that makes contact with the front surface land portion is smaller than a profile of the lower fillet portion that makes contact with the back surface land portion, and is not less than an inner diameter of the through hole.
(Embodiment 2) The connection structure according to embodiment 1, wherein a profile of the front surface land portion is smaller than a profile of the back surface land portion.
(Embodiment 3) The connection structure according to embodiment 1, wherein a peripheral edge of the front surface land portion is covered with a solder resist.
(Embodiment 4) The connection structure according to any one of embodiments 1 to 3, wherein the wall surface land portion has a tapered portion at an end of the wall surface connecting to the front surface of the board.
(Embodiment 5) The connection structure according to any one of embodiments 1 to 4, wherein another through hole is provided to penetrate the board at a position near said former through hole, while said land is formed so as to cover a wall surface of said another through hole and connect between the through holes on the front surface and the back surface of the board, while a portion of the land connecting the through holes on the front surface of the board is covered with the solder resist.
(Embodiment 6) A connection structure comprising a board having a front surface and a back surface with a through hole formed to penetrate through the board; a land including a wall surface land portion formed on a wall surface of the through hole, a front surface land portion formed on the front surface of the board around the through hole and a back surface land portion formed on the back surface of the board around the through hole; a lead extending from an electronic component and disposed so as to penetrate the through hole from the front surface to the back surface of the board; and a fillet made of a solder material by flow soldering so as to connect the land and the lead, the fillet including an upper fillet portion located on the front surface of the board and a lower fillet portion located on the back surface of the board, the connection structure being constructed so as to suppress temperature rise in the front surface land portion.
(Embodiment 7) The connection structure according to embodiment 6, wherein a profile of the upper fillet portion that makes contact with the front surface land portion is smaller than a profile of the front surface land portion.
(Embodiment 8) The connection structure according to embodiment 6 or 7, wherein the board includes a heat sink formed in contact with the front surface land portion.
(Embodiment 9) The connection structure according to any one of embodiments 6 to 8, wherein a notch is formed in the wall surface land portion.
(Embodiment 10) A connection structure comprising a board having a front surface and a back surface with a through hole formed to penetrate through the board; a land including a wall surface land portion formed on a wall surface of the through hole, a front surface land portion formed on the front surface of the board around the through hole and a back surface land portion formed on the back surface of the board around the through hole; a lead extending from an electronic component and disposed so as to penetrate the through hole from the front surface to the back surface of the board; and a fillet made of a solder material by flow soldering so as to connect the land and the lead, the fillet including an upper fillet portion located on the front surface of the board and a lower fillet portion located on the back surface of the board, wherein at least one projection is provided on at least one of the front surface land portion and the back surface land portion.
(Embodiment 11) The connection structure according to embodiment 10, wherein said projection is located at a peripheral edge of at least one of the front surface land portion and the back surface land portion.
(Embodiment 12) A connection structure comprising a board having a front surface and a back surface with a through hole formed to penetrate through the board; a land including a wall surface land portion formed on a wall surface of the through hole, a front surface land portion formed on the front surface of the board around the through hole and a back surface land portion formed on the back surface of the board around the through hole; a lead extending from an electronic component and disposed so as to penetrate the through hole from the front surface to the back surface of the board; and a fillet made of a solder material by flow soldering so as to connect the land and the lead, the fillet including an upper fillet portion located on the front surface of the board and a lower fillet portion located on the back surface of the board, wherein a portion of the lead that is not disposed inside the through hole and is located above the front surface of the board is covered with a solder resist so as to restrict a height of the fillet with respect to the board surface by the solder resist.
(Embodiment 13) A connection structure comprising a board having a front surface and a back surface with a through hole formed to penetrate through the board; a land including a wall surface land portion formed on a wall surface of the through hole, a front surface land portion formed on the front surface of the board around the through hole and a back surface land portion formed on the back surface of the board around the through hole; a lead extending from an electronic component and disposed so as to penetrate the through hole from the front surface to the back surface of the board; and a fillet made of a solder material by flow soldering so as to connect the land and the lead, the fillet including an upper fillet portion located on the front surface of the board and a lower fillet portion located on the back surface of the board, wherein the land is covered with a film (for example, a metal film).
(Embodiment 14) The connection structure according to embodiment 13, wherein the film is made of a metal selected from the group consisting of Sn, Snxe2x80x94Cu and Snxe2x80x94Ag.
(Embodiment 15) An electronic circuit board comprising the connection structure of any one of embodiments 1 to 14.
(Embodiment 16) The connection structure according to any one of embodiments 1 to 14, wherein all of metal elements that constitute the solder material and all of alloys made of two or more of said metal elements have melting points not lower than a melting point of said solder material.
(Embodiment 17) The connection structure according to embodiment 16, wherein the solder material is selected from the group consisting of Snxe2x80x94Cu, Snxe2x80x94Agxe2x80x94Cu, Snxe2x80x94Ag and Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu.
(Embodiment 18) The connection structure according to embodiment 16 or 17, wherein the lead includes a base member and a plating member that covers said base member, and all of metal elements that constitute the solder material and the plating member and all of alloys made of two or more of said metal elements in all possible combinations have melting points not lower than the melting point of said solder material.
(Embodiment 19) The connection structure according to embodiment 18, wherein the plating member is made of a metal selected from the group consisting of Sn, Snxe2x80x94Cu and Snxe2x80x94Ag.
(Embodiment 20) The connection structure according to embodiment 18 or 19, wherein all of metal elements that constitute the solder material, the base member and the plating member, and all of alloys made of two or more of said metal elements in all possible combinations have melting points not lower than the melting point of said solder material.
(Embodiment 21) The connection structure according to any one of embodiments 18 to 20, wherein the base member is made of a material selected from the group consisting of Cu, Fe and an Fexe2x80x94Cu alloy.
(Embodiment 22) The connection structure according to embodiment 16 or 17, wherein the lead includes a base member and a plating member that covers said base member, and all of metal elements that constitute the solder material, the base member and the plating member and all of alloys made of two or more of said metal elements in all possible combinations have melting points not lower than the melting point of said solder material, and the lead is provided with means for preventing the metal elements that constitute the base member from melting into the fillet.
(Embodiment 23) The connection structure according to embodiment 22, wherein the base member is made of a Zn based alloy and said means is an underlaying plating member made of Ni disposed between the base member and the plating member.
(Embodiment 24) An electronic circuit board comprising the connection structure of any one of embodiments 16 to 23.
(Embodiment 25) A connection structure comprising a board having a front surface and a back surface with a through hole formed to penetrate through the board; a land formed on a wall surface of the through hole, the front surface and the back surface of the board around the through hole; a lead extending from an electronic component and disposed in the through hole; and a fillet made of a solder material by flow soldering so as to connect the land and the lead, wherein all of metal elements that constitute the solder material and all of alloys made of two or more of said metal elements in all possible combinations have melting points not lower than a melting point of said solder material.
(Embodiment 26) The connection structure according to embodiment 25, wherein the solder material is selected from the group consisting of Snxe2x80x94Cu, Snxe2x80x94Agxe2x80x94Cu, Snxe2x80x94Ag and Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu.
(Embodiment 27) The connection structure according to embodiment 25 or 26, wherein the lead includes a base member and a plating member that covers said base member, and all of metal elements that constitute the solder material and the plating member, and all of alloys made of two or more of said metal elements in all possible combinations have melting points not lower than the melting point of said solder material.
(Embodiment 28) The connection structure according to embodiment 27, wherein the plating member is made of a metal selected from the group consisting of Sn, Snxe2x80x94Cu and Snxe2x80x94Ag.
(Embodiment 29) The connection structure according to embodiment 27 or 28, wherein all of metal elements that constitute the solder material, the base member and the plating member, and all of alloys made of two or more of said metal elements in all possible combinations have melting points not lower than the melting point of said solder material.
(Embodiment 30) The connection structure according to any one of embodiments 27 to 29, wherein the base member is made of a material selected from the group consisting of Cu, Fe and an Fexe2x80x94Cu alloy.
(Embodiment 31) The connection structure according to embodiment 25 or 26, wherein the lead includes a base member and a plating member that covers said base member, and all of metal elements that constitute the solder material, the base member and the plating member, and all of alloys made of two or more of said metal elements in all possible combinations have melting points not lower than the melting point of said solder material, while the lead is provided with means for preventing the metal element that constitutes the base member from melting into the fillet.
(Embodiment 32) The connection structure according to embodiment 31, wherein the base member is made of a Zn based alloy and said means is an underlaying plating member made of Ni disposed between the base member and the plating member.
(Embodiment 33) An electronic circuit board comprising the connection structure of any one of embodiments 25 to 32.